<template>
    <div class="list">
        <li class="item" v-for="item of letters" :key="item"
            :ref="item"
            @click="handleLetterClick"
            @touchstart.prevent="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
        >{{item}}</li>
    </div>
</template>

<script>
    export default {
        name:"CityAlphabet",
        data() {
            return {
                touchStatus: false,
                 startY: 0,
                timer: null
            }
        },
        computed: {
            letters(){
                const letters = []
                for (let i in this.cities){
                    letters.push(i)
                }
                return letters
            }
        },
        updated () {
            this.startY = this.$refs['A'][0].offsetTop
        },
        methods: {
            handleLetterClick(e){
                this.$emit('change',e.target.innerText)
            },
            handleTouchStart () {
                this.touchStatus = true
            },
            handleTouchMove (e) {
                 if (this.touchStatus) {
                    if (this.timer) {
                    clearTimeout(this.timer)
                }
                this.timer = setTimeout(() => {
                const touchY = e.touches[0].clientY -165
                const index = Math.floor((touchY - this.startY) / 20)
                if (index >= 0 && index < this.letters.length) {
                    this.$emit('change', this.letters[index])
                 }
        }, 16)
      }   
             },
            handleTouchEnd () {
                this.touchStatus = false
            }
        },
        props:{
            cities:Object
        }
    }
</script>

<style lang="stylus" scoped>
@import '~@style/varibles.styl'
    .list 
        display flex
        flex-direction column
        justify-content center
        position absolute
        top 1.78rem
        right 0
        bottom 0
        width 0.4rem
        .item
            text-align center
            line-height .4rem
            color $bgColor
</style>